The purpose of this proposal is to develop and implement a randomized controlled clinical trial to determine the effect of a low fat diet (20% of dietary calories) on breast cancer incidence. The study population at this institution will consist of at least two hunderd women age 45-70 whose annual risk is estimated to be at least 1%. The study period will last five years. The aims of the study include: 1) Recruitment of eligible patients from a population of women attending the Strax Cancer Detection Institute or relatives of breast cancer patients that have been or are being treated by the clinical service of the Breast Cancer Division of the Comprehensive Cancer Center for the State of Florida. 2) Baseline and annual physical examination, mammography, dietary habits and associated behavior. Maintain a high level of quality control and comparability between assessments. 3) Randomization of the study subjects into experimental (intervention) and control groups in such a way as to eliminate contamination. 4) Administration of the nutritional and behavior modification intervention for the experimental group. Study of psychosocial variables predictive of regimen adherence. 5) Objective assessment of dietary adherence to the intervention protocol by nutritional history, biochemical lipid parameters and adipose tissue fatty acid analysis. In this proposal, particular attention has been paid to recruitment, contamination of controls, nutritional intervention and behavior modification as we feel that these factors must be dealt with correctly to assure the success of the study. The data will be collected, edited and transmitted to the National Cancer Institute.